Space Plane: Technology In Search of a Mission? (Part 1)

The “space plane” concept of being able to launch an unpiloted spacecraft onto an orbital or suborbital path and have it return to Earth like an airplane has been discussed for many years. The history of U.S. development of a space plane shows that the program has shifted between civil and military oversight, and that the motivations for developing such a system have also changed significantly over time.

Now that the X-37B—a small prototype space plane—has had its first test launch and is orbiting the Earth, it’s worth asking: what tasks might a space plane do better, more efficiently, and cheaper than other systems?

Following our earlier post on this topic, we have continued to look into the details of this question. Our conclusion is that it is difficult to find a mission for which the space plane makes sense. The reason is that you pay a big price for bringing the system back to Earth to reuse it, rather than developing a simpler and lighter system.

In particular, compared to alternate systems, a space plane doesn’t make much sense for military applications such as deploying space weapons or providing additional surveillance capabilities. So the space plane itself shouldn’t be seen as a threat by other countries, even if these applications are.

For example, for reasons we discuss below, concerns about the space plane as a “space bomber” appear unfounded. If the United States decided to develop the capability to quickly strike ground targets over long ranges, using a space plane as part of that system would almost certainly reduce its strike capability.

At the same time, compared to alternative systems a space plane does not appear to be very useful for non-military other missions, either.

That’s not to say we can’t think of something that the X-37B or its successor could do. It means that in almost all of those cases we can identify a better, more efficient, and/or cheaper way of doing each of those tasks. It may well be that with money being spent on space-plane development, NASA or the Pentagon has neglected to develop some of these alternative systems, and that inertia will help keep the space-plane concept alive, even as a sub-optimal solution.

But as we show below, the benefits of these alternatives are significant enough that the administration and congress should review its commitment to the space plane program. At the very least, it should be clear about why it is spending money on its development and should not see it as a solution to a wide array of problems.

What What Does a Space Plane Offer?

The X-37B is called an Orbital Test Vehicle (OTV), and is being used to develop technology for a reusable space vehicle that can demonstrate autonomous atmospheric entry and runway landing. These technologies could presumably be used either for a small vehicle the size of the current X-37B or a larger space plane.

Here is our main takeaway:

The only unique capability of a space plane appears to be its ability to return from orbit and land autonomously on a runway. Other systems that do not return to Earth can be used to carry payloads into orbit, maneuver in space, rendezvous with satellites, release multiple payloads, etc., and do so at much less cost.

Building this return capability into the space plane adds tons of extra mass compared to maneuvering spacecraft that are not designed to return to Earth, since the space plane requires wings as well as additional structure and heat-shielding to withstand the rigors of atmospheric reentry. That large mass penalty makes it more difficult and expensive to get a space plane and its payload into orbit and reduces the amount of maneuvering that it can do with a given amount of fuel.

So unless there is very compelling reason to bring something back from orbit, using a space plane would be much more expensive than other means of carrying out the mission.

Even in the case of bringing people back to Earth, it is useful to note that the successor the Space Shuttle is being designed as a reusable capsule without wings, which can land on the ground or water using parachutes.

To better understand our conclusion, consider these examples:

(1) Because of the space plane’s large mass, using it to deliver satellites or other objects to orbit is more costly and requires a much larger launch vehicle than using other launch methods.

Development of a space plane is often confused with the development of reusable launch vehicles. But the X-37B is dead-weight during launch, so it does not help get payloads into orbit. Unlike the Space Shuttle, it does not use its engines during launch.

The X-37B has a launch mass of 5 metric tons (t) without a payload. This large mass requires a significantly larger launch vehicle than would other ways of launching satellites. For example, even with no payload, the X-37B could not be launched on a Delta II launcher, which we show below is routinely used to launch multiple satellites. Indeed, the Atlas V launcher used to place the X-37B into orbit on April 22 is twice as massive as the Delta II that is used to lift a maneuvering rocket stage carrying four half-ton Globalstar satellites into orbit.

Keep in mind that a general rule of thumb is that it costs roughly $20,000 per kilogram to launch an object into low Earth orbit, so there is a big incentive to minimize the mass that is launched.

Despite the large size of the X-37B, it has limited ability to carry a payload. Scaling from the payload capacity of the Space Shuttle, we estimate that the maximum payload for the X-37B is likely under 1 ton. In addition, the payload bay is quite small—7 feet (2.1 m) long by 4 feet (1.2 m) in diameter, so it can only accommodate small payloads (it is too small to accommodate even a single Globalstar satellite). Scaling the system up to carry larger payloads would also increase the mass of the space plane and further increase launch costs.

The space plane also doesn’t help with developing prompt, on-demand launch services, which typically focus on developing small launchers to lift small objects into space without the cost and infrastructure required for a large launch vehicle. Moreover, the reusability of the space plane itself doesn’t appear to be a significant advantage if it is being launched on expendable launchers.

One argument for the space plane is that it would provide a standard interface to mount different satellites and payloads to be sent into space. But small, maneuvering rocket stages—sometimes called “buses”—can be designed with standard interfaces for mounting payloads without building in the large mass needed to bring them back to Earth after they were used.

(2) For missions that require maneuvering in space, other systems exist that are cheaper and have more ability to maneuver.

Maneuvering buses are routinely used for putting multiple satellites into different orbits on a single launch, and do so very efficiently. These buses are relatively inexpensive and are not intended to be reused.

Such maneuvering buses could also be used to carry and reposition sensors to augment current surveillance capability when needed.

Because these maneuvering stages can be several tons lighter than a space plane, they can offer the same amount of maneuverability as a space plane for much lower mass. This allows them to be launched on a smaller launch vehicle at significantly lower cost. Alternately, such buses could give much greater maneuverability for the same launch mass.

Some details

The maneuverability of a spacecraft with a given amount of fuel can be quantified by the total amount it can change its velocity while in orbit. This is called “delta velocity,” or ΔV.

The X-37B, without payload, has a liftoff mass of about 5 tons, about 1.5 tons of which are maneuvering fuel. It therefore has a total maneuverability of about ΔV = 1 km/s or less, even without a payload. This is only about one-third of the maneuverability that has been discussed in the past as a goal for the space plane.

Moreover, roughly a quarter to a third of that ΔV cannot be used for on-orbit maneuvering but must be reserved to de-orbit the X-37B to bring it back to Earth. That reentry fuel would have a mass of more than a quarter ton, and is mass that must be carried into orbit and moved around with every on-orbit maneuver (i.e., maneuvering that quarter-ton of deorbit fuel itself costs fuel.).

In contrast, a maneuvering bus with the same maneuvering capability and no requirement to return to Earth could have a much smaller mass.

(a) One example is the MIRV bus for the U.S. Trident II ballistic missile. The bus is designed to carry up to eight Mk-5 warheads, each with a mass of 180 kg (for a total payload of 1.44 t). The bus releases them one at a time, maneuvering between each release to put them on different trajectories, in a process similar to releasing multiple satellites into different orbits. This bus is believed to have a fueled mass of just over 1 ton and a total maneuverability of roughly ΔV = 1 km/s. This represents greater maneuverability than the X-37B with a larger payload capacity, for about one-fifth of the mass.

(b) Another example of an efficient maneuvering system that is routinely used to launch multiple satellites is the U.S. Delta II launcher. The Delta II has been frequently used in a two-stage configuration (with strap-on boosters) to place four Globalstar satellites or five Iridium satellites into orbit on a single launch. The second stage, which doubles as a maneuvering bus that dispenses the satellites in their various orbits, has a dry mass of less than a ton. It can carry up to roughly 6 tons of fuel if a lot of maneuverability is needed, which would give it a ΔV greater than 3 km/s while carrying a 2-ton payload.

(The first generation Globalstar satellites had a mass of 450 kg, so four Globalstar satellites would give a payload of 1.8 tons. The Globalstar constellation has 6 satellites in each of 8 orbital planes, and the satellites orbit at an altitude of 1,414 km at 52 degrees inclination. The Iridium satellites have a mass of 690-725 kg, depending on the version, and orbit at 781 km in orbits inclined at 86.4 degrees. So five Iridium satellites would give a payload of 3.5 tons. The Iridium constellation consists of 11 satellites in each of 6 orbital planes, for a total of 66 satellites.)

The version of the Delta II used to launch four Globalstar satellites has a launch mass of 166 t. The version of the Delta II used to launch five Iridium satellites has a launch mass of 233 t. The cost of a Delta II launch is estimated to be $50-60 million.

In contrast, the X-37B, presumably carrying little or no payload, was reportedly launched on an Atlas V 501 launcher, which has a launch mass of 330 t. Therefore, just getting the X-37B into orbit required a launcher twice as massive as the Delta II used to launch multiple Globalstar satellites. The X-37B on its own was already too heavy to launch on the Delta II. The launch of an Atlas V (500 series) is estimated to be more than $100 million, which is twice that of the Delta II. We would expect comparable costs for a launch on a Delta IV launcher.

(c) An additional example is the launch of multiple Iridium satellites by China in 1997-8. Using the Long March (CZ)-2C launcher, China conducted several launches of a maneuvering bus that placed two Iridium satellites in orbit on each launch. The CZ-2C had a liftoff mass of 191 tons—comparable to the Delta II. We have seen the bus used to launch the Iridiums in a missile museum in Beijing, and estimating from its size and the lift capability of the CZ-2C, its fueled mass must be roughly a ton or less.

We also compare launching multiple satellites from a space plane versus several small launchers in the Physics of Space Security.

(3) These other maneuvering systems can rendezvous with satellites—to inspect or refuel them—can carry surveillance sensors, or can be used to carry systems being space-tested into orbit.

While some people discuss putting sensors on a system like X-37B and using it either to rendezvous with and inspect an orbiting satellite, or to look at the ground, there are other ways to do this more cheaply. Small maneuvering satellites or buses could be equipped with sensors to do these missions. They could be designed with standard interfaces that would allow payloads—either sensor packages or experiments—to be readily mounted on them. Because they would not be designed to land after use, their mass could be much smaller than the space plane. This would make launching easier and could give them much greater maneuverability, and would also make them smaller and intrinsically stealthier, which for some missions may be desirable. The Mitech satellites in GEO that were used to inspect the failed U.S. DSP-23 satellite after are an example.

As a result, surveillance and inspection do not appear to be good missions for the space plane.

(4) For the reasons given above, the X-37B also does not make sense for delivering weapons into orbit.

The X-37B is a poor choice for placing weapons into orbit, including hypersonic strike vehicles intended for a conventional Global Strike mission. It is also unlikely for the space plane itself to perform the strike mission, as it is much bigger and slower (and much more expensive) than a re-entry vehicle or common aero vehicle.

What we have discussed in this post is why a space plane is poorly suited to missions that don’t require returning something from orbit back to Earth. In particular, the mass required to give a space plane the ability to return to Earth strongly limits its ability to maneuver on orbit and makes it a poor substitute for less massive, expendable means for deploying multiple satellites.

In a subsequent post, we will look at whether the expense of the space plane is justified by the need to return objects from orbit.

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David Wright, physicist and co-director of the UCS Global Security Program, is an established expert on the technical aspects of arms control, particularly those related to missile defense systems, missile proliferation, and space weapons.